1. Field of the Invention
The invention relates to a circuit arrangement for detecting the capacitance or capacitance change of a capacitive circuit element or component. More specifically, the invention relates to a circuit arrangement for detecting the capacitance or capacitance change of a capacitive circuit element or component, including: a voltage source, at least one charging switch, at least one recharging switch, a control device which controls the charging switch and the recharging switch and which contains a clock generator, a storage capacitor, and an evaluation circuit connected to the storage capacitor.
2. Description of Related Art
Within the framework of the invention, “capacitance” means the capacitance value of a capacitive circuit element or component; a “capacitance change” consequently means a change of the capacitance value of a capacitive circuit element or component. “Detection” of the capacitance or capacitance change within the framework of the invention means both only qualitative detection and also quantitative detection. A “capacitive circuit element or component” within the framework of the invention means any circuit element and any component which has capacitive property called a capacitance. A “capacitive circuit element or component” within the framework of the invention however also means the electrode of a capacitive proximity switch in interaction with an influencing body. A “capacitive circuit element or component” within the framework of the invention also means, for example, the capacitance which represents conductive lines which capacitively interact with one another. Instead of using the term “capacitive circuit element or component”, a sensor capacitor is used herein in describing the prior art and the present invention, without being associated with the limitation to a capacitor in the narrower sense.
Within the framework of the invention a “voltage source” means both an internal voltage source overall and also a terminal for an external voltage source.
The circuit arrangement underlying the present invention works according to the so-called “charge transfer principle,” also called “charge transfer sensing,” and is known, for example, from German patent 197 01 899 and 197 44 152 and their counterpart U.S. Pat. No. 6,194,903, which are incorporated herein by reference, and will be explained below in conjunction the drawing of FIG. 1.
FIG. 1 shows, in principle, an embodiment of a known circuit arrangement for detection, specifically for quantitative detection for the measurement of the capacitance of a sensor capacitor 1. The sensor capacitor 1 stands for a capacitive circuit element or component. The circuit arrangement includes a voltage source 2, wherein the expression “voltage source” means both a voltage source implemented within the circuit arrangement and also a terminal for this voltage source as previously mentioned. In the circuit arrangement shown in FIG. 1, there is only one terminal for an internal or external voltage source; nevertheless, this terminal for a voltage source is always labeled a voltage source 2 below.
The circuit arrangement shown in FIG. 1 further includes a charging switch 3, a recharging switch 4, a control device 5 which alternately controls the charging switch 3 and the recharging switch 4 and which contains preferably a clock generator (not shown), a storage capacitor 6, and an evaluation circuit 7 connected to the storage capacitor 6. In this embodiment, the control device 5 and the evaluation circuit 7 are combined into a control and evaluation unit 8.
As shown in FIG. 1, the voltage source 2 is connected, by closing the charging switch 3, to the first electrode 9 of the sensor capacitor 1, and the second electrode 10 of the sensor capacitor 1 is connected to the terminal of the voltage source 2, wherein the terminal has an opposite polarity from charging switch 3. In this embodiment, the connection of the second electrode 10 of the sensor capacitor 1 to the terminal of the voltage source 2, is to a common potential, specifically the ground potential 11. The above described connection of the sensor capacitor 1, the voltage source 2 and the charging switch 3 leads to the sensor capacitor 1 being charged by the voltage source 2 when the charging switch 3 is closed.
As can further be seen from FIG. 1, electrode 12 of the storage capacitor 6 is connected to the electrode 9 of the storage capacitor 1, which is connected to the charging switch 3, and the second electrode 13 of the storage capacitor 6 is connected, by closing the recharging switch 4, to the second electrode 10 of the sensor capacitor 1. The above described connection of the sensor capacitor 1, the recharging switch 4 and the storage capacitor 6 allows the sensor capacitor 1 to discharge onto the storage capacitor 6 with the charging switch 3 opened and the recharging switch 4 closed so as the charge stored in the sensor capacitor 1 is recharged or transferred onto the storage capacitor 6.
Finally, FIG. 1 shows that a discharging switch 14 connects the electrode 12 of the storage capacitor 6 and the electrode 9 of the sensor capacitor, with the ground potential 11. Before starting a measurement of the capacitance of the sensor capacitor 1, the storage capacitor 6 is first discharged in a defined manner by both the recharging switch 4 and also the discharging switch 14 being closed. When the recharging switch 4 and the discharging switch 14 are closed, the storage capacitor 6 is shorted via the recharging switch 4, the ground potential 11 and the discharging switch 14.
As known in the prior art about the “charge transfer principle” or “charge transfer sensing,” the capacitance of the sensor capacitor 1 can be determined by the evaluation circuit 7 from the voltage on the storage capacitor 6 after a certain number of charging and discharging cycles, under the assumption that the voltage of the voltage source 2 and the capacitance of the storage capacitor 6 are known. This is because the voltage on the capacitor is proportional to its charge, as is generally known.
From the known voltage of the voltage source 2, the capacitance of the storage capacitor 6 and the number of charging and discharging cycles, hence, the capacitance of the sensor capacitor 1 can either be determined by the number of charging and recharging cycles, which is necessary for a certain voltage on the storage capacitor 6, or by the voltage on the storage capacitor 6 for a certain number of charging and recharging cycles.
The known circuit arrangements which work according to the charge transfer principle (“charge transfer sensing”) have proven themselves in practice and are therefore extensively implemented. However, they are subject to one defect, specifically sensitivity to LF noise voltages. These LF noise voltages can adulterate the measurement result, as this will be explained below in conjunction with FIG. 2. The circuit arrangement shown in FIG. 2 corresponds completely to the circuit arrangement shown in FIG. 1, only with an addition of a LF noise voltage source 15.
It is assumed that the voltage source 2 provides an operating voltage UB of 5 V and the LF noise voltage source 15 generates a LF noise voltage US with an instantaneous value of 1 V. The LF noise voltage potential PS should be positive on the second electrode 10 of the sensor capacitor 1 relative to the ground potential 11 for the observation instant; therefore, on the second electrode 10 of the sensor capacitor 1, there is a LF noise voltage PS of 1 V.
Furthermore, it is assumed that, before the start of the measurement cycle, the changeover switch 4 and the discharge switch 14 have been closed so that the storage capacitor 6 has been discharged in a defined manner, and that then the discharge switch 14 remains opened during the measurement cycle.
At this point, a first charging and recharging cycle will take place. First, the charging switch 3 is closed for a charging time tL which is sufficient for charging of the sensor capacitor 1, and then, after the charging switch 3 has been re-opened, the recharging switch 4 is closed for a recharging time tU which is sufficient for recharging the charge stored initially in the sensor capacitor 1 into the storage capacitor 6.
For the examination which now follows, it must be considered that the charging and recharging cycle time tLUZ of the charging and recharging cycle, which is generally slightly greater than the sum of the charging time tL and the recharging time tUr, is small compared to the period length tS of the LF noise voltage US, but that the measurement time tMZ, therefore the number of charging and recharging cycles which is determined from the one measurement cycle, is small compared to the period length tS of the LF noise voltage US.
For the circuit arrangement shown in FIG. 1, without the LF noise voltage source 15 shown in FIG. 2, it holds that the electrode 9 of the sensor capacitor 1 is at the operating voltage potential PUB of the voltage source 2 at 5 V, and the second electrode 10 is at the ground potential 11 at 0 V, and that the charge of the sensor capacitor 1 which is recharged into the storage capacitor 6 after closing the recharging switch 4 results from the capacitance of the sensor capacitor 1 and the operating voltage UB of 5 V.
Recharging of the charge stored after charging in the sensor capacitor 1 onto the storage capacitor 6 leads to the voltage on the storage capacitor 6, which was 0 V at the start of recharging, to be increasing both during any recharging and also for any recharging which follows the first recharging. This also means mainly that during the second recharging less charge is recharged or transferred from the sensor capacitor 1 to the storage capacitor 6 than in the first recharging, during the third recharging less than in the second recharging, during the fourth recharging less than in the third recharging, and etc.
What is explained above must be considered in the determination of the capacitance of the sensor capacitor 1, from the voltage of the voltage source 2, the capacitance of the storage capacitor 6, from the number of charging and recharging cycles necessary for a certain voltage on the storage capacitor 6, and from the voltage on the storage capacitor 6 at a certain number of charging and recharging cycles.
It has been pointed out that, when the capacitance of the storage capacitor 6 is very large compared to the capacitance of the sensor capacitor 1, when the charging time tL is very small and when the measurement time tMZ is very small, the number of charging and recharging cycles which determines one measurement cycle is small. In such case, the above effects can be ignored. In practice, however, what was explained above cannot be ignored, but rather is considered in the determination of the capacitance of the sensor capacitor 1.
Now, if the LF voltage source 15 shown in FIG. 2 with a LF noise voltage US, as assumed above, with an instantaneous value of 1 V at the start of a recharging cycle, is active, and the instantaneous value of the LF noise voltage US during the recharging cycle increases, it holds that not only the charge stored previously in the sensor capacitor 1 is recharged into the storage capacitor 6, but also a current caused by the time change of the LF noise voltage US also flows through the sensor capacitor 1 into the storage capacitor 6, therefore, an addition charge is transported into the storage capacitor 6. The current which is caused by the time change of the LF noise voltage US and which flows through the sensor capacitor 1 into the storage capacitor 6, will hereinafter be called the LF noise voltage fault current, and the charge thus transported in addition into the storage capacitor 6 will be called the LF noise voltage fault charge. The measurement result which can be determined from the voltage on the storage capacitor 6 after a certain number of charging and recharging cycles is therefore adulterated by the time change of the LF noise voltage US because the voltage which arises on the storage capacitor 6 is no longer dependent only on the voltage of the voltage source 2 operating voltage UB. Accordingly, the number of charging and recharging cycles and the capacitance of the storage capacitor 6 is now also dependent on the unwanted LF noise voltage US with an unknown magnitude and the time change of the LF noise voltage US during one recharging cycle, specifically on the LF noise voltage fault charge which has been caused thereby.
It previously mentioned, what is meant within the framework of the invention by a “capacitive circuit element or component” is a sensor capacitor. Consequently, instead of a “capacitive noise voltage compensation element,” a noise voltage compensation capacitor will always be addressed below, and this should not be associated with a limitation to a capacitor in the narrower sense.